1. Field of the Invention
The present invention relates to a method for reproducing an optical recording medium, and particularly, to a method for reproducing an optical recording medium capable of distinguishing between near-field and far-field optical recording media and interchangeably reproducing both types of optical recording media.
2. Description of the Related Art
In the current field of optical recording media, various technologies and recording media are being developed. The current optical recording media may be categorized into a far-field type and a near field type.
The near-field optical recording medium and far-field optical recording medium will now be described schematically.
FIG. 1 is a block diagram of an optical pick-up of a reproducing device that may reproduce a far-field optical recording medium, illustrating a configuration of an optical pick-up, which may reproduce a DVD.
The optical pick-up includes a laser diode 101, a collimator lens 102, a beam splitter 103, an objective lens 104, a cylindrical lens 105 and a photo diode 106.
In general, light generated from the laser diode 101 is emitted with oval intensity distribution. In order to efficiently concentrate light to the objective lens 104 having a circular incident surface, the intensity distribution of the emitted light is shaped into a circular form by using a beam shaping prism or the like.
The beam splitter 103 is used to detect light reflected by the optical disc.
When light transmitted through the beam splitter 103 passes through a λ/4 wavelength plate (not shown), linear polarized light is changed into circular polarized light by the λ/4 wavelength plate (not shown) and is made incident upon the objective lens 104.
Light incident upon the objective lens 104 is reflected by the optical disc 107 and is transmitted through the objective lens 104 again. Here, such reflection is made between the air having a low refractive index and the optical disc 107 having a high refractive index, so that the phase of the light is changed by 180°. In this case, the light shaped into the circular polarized light by the λ/4 wavelength plate (not shown) is changed into linear polarized light while transmitting the λ/4 wavelength plate (not shown) again, but its polarization direction is changed to be perpendicular to the initial polarization direction. Accordingly, most of light is reflected by the beam splitter 103 and is made incident upon the photo diode 106, thereby detecting an optical signal.
Research on various methods for increasing recording capacity of an optical recording medium is being conducted, and development of a Blueray disc (BD) is being made as well as a CD and DVD.
In order to increase the recording capacity of an optical recording medium, methods of reducing a wavelength of a laser beam or increasing the number of apertures (NA) of an objective lens are being studied.
However, when it comes to the Blueray disc, there is limit to the method of reducing a wavelength, and in the existing far-field method, increasing the number of apertures of an objective lens cannot contribute to the improvement of the recording capacity.
A near-field type using a recently-developed solid immersion lens (SIL) is drawing much attention for its possibility of remarkably increasing recording capacity as a hemispherical SIL made of a medium having a refractive index much higher than 1 is disposed under an objective lens, thereby allowing the number of apertures substantially to be n*NA which is much higher than 1
FIG. 2. is a view of an optical pick-up that can reproduce an optical recording medium supporting a near-field.
Unlike the far-field optical recording medium, the optical pick-up for reproducing a near-field optical recording medium includes a solid immersion lens (SIL) 202.
The principle of the optical pick-up using the SIL 202 will now be described schematically.
Referring to FIG. 2, when a laser is concentrated to the SIL 202 by an objective lens 201, a focus is formed at a bottom inside the SIL 202 with a high refractive index, and its focus spot diameter is decreased in proportional to a refractive index of the SIL 202, so that a spot size smaller than a diffraction limit is obtained.
In general, an optical system includes two types of lenses. One is a free focus lens loaded on a surface of a lift type head and corresponds to an objective lens 202 in the related art optical pick-up. The other one is a hemispherical lens called an SIL 202. The SIL 202 is made of a material with a high refractive index (n) such as glass or the like and is loaded on a lower surface of the lift type head. Also, a coil is disposed to surround the SIL 202.
When a new type of reproducing device of an optical recording medium is developed, interchangeability with the existing type should be considered to be important. If the device has no interchangeability with the exiting type in a market, users may disregard the new type of product and it may become difficult for the new type product to enter the market.
Also, although a reproducing device with interchangeability is developed, a technology of distinguishing between the different types of optical recording media may become an issue.
Particularly, a distance between a bottom of the SIL and a surface of the optical recording medium of the optical pick-up is in tens of nanometers. For this reason, if the kind of inserted media is not accurately recognized in operation of the reproducing device, the optical pick-up or the optical recording medium may be damaged.
In order to avoid such damage, a new method is being demanded for speedily determining the type of optical recording medium in a very simple manner before the operation of a reproducing device having interchangeability is started.